The use of peroxide as a sanitizer or disinfectant for various types of equipment, like food processing equipment and medical equipment, such as a hemodialysis unit, is common. Because the amount of peroxide in an aqueous solution relates directly to the disinfecting or sanitizing activity thereof, a test which rapidly and accurately measures peroxide concentration is important.
The use of a peroxide, like hydrogen peroxide or peracetic acid, as a disinfectant for medical equipment is widespread because of its low cost, convenience, and effectiveness as an antiseptic agent in relatively low concentrations. For example, peroxides are used as a disinfectant in a substantial number of hemodialysis centers. Peroxides are used in hemodialysis centers to sanitize hemodialysis units because peroxides are an effective and economical sanitizing agent. It also is important to clean and disinfect a hemodialysis unit between each dialysis session to prevent pathogen contamination from patient to patient.
When a sanitizing solution is used in medical or food processing equipment, two critical peroxide levels must be monitored. First, the peroxide concentration must be sufficiently high to perform a sanitizing or disinfecting function, i.e., at least about 500 ppm (parts per million) peroxide is needed to effectively sanitize equipment. During the sanitizing process, the sanitizing solution is assayed periodically to ensure that sufficient peroxide is present to sanitize the equipment.
After the sanitizing function is completed, and before reuse, the equipment is rinsed with water to flush residual peroxide from the equipment. The rinse water also is assayed for peroxide to ensure that the level of residual peroxide is below the maximum allowable level.
Presently, there is one type of commercial assay system for assaying hemodialysis units for peroxide concentration. The assay is semiquantitative and merely indicates that sufficient peroxide is present to sanitize the equipment, or that the peroxide is below this sanitizing level. No other quantitative information is available from the assay. This assay relies on a color transition formed by a starch iodine complex.
A starch-iodide solution had been used as an indicator for oxidation-reduction (redox) titrations for decades. The starch iodine complex has a sharp and intense color transition, turning from colorless for the iodide ion to dark blue for the starch iodine complex. A starch iodine color transition, therefore, is used mainly as an end point indicator. The color intensity of the starch iodine complex formation is so great that the color transition from iodide to iodine does not allow a quantitative distinction between different concentrations of an oxidant, like peroxide, present in a test sample. Consequently, there was little to no impetus to use formation of a starch iodine complex in a quantitative calorimetric determination of an oxidant in a test sample, especially at high oxidant concentrations.
To be useful in a quantitative assay, color transitions must be differentiable and related to the concentration of the analyte of interest. Therefore, it is necessary to find a color-forming complex that undergoes a color transition of weaker intensity than the starch iodine complex. The identity of this less intensely colored complex has evaded workers in the art, especially with respect to an assay for a peroxide in a high concentration range of about 0.1% to greater than about 4% by weight of the test sample.
The search for a compound that binds to iodine, and forms a colored complex suitable for use in a quantitative assay for an oxidant, like peroxide, has not been successful until the present disclosure. For example, M. M. Zwick, Journal of Applied Polymer Science, Vol. 9, pp. 2393-2424 (1965), discloses formation of blue color complex of iodine and polyvinylalcohol. The Zwick publication discloses a structural requirement and mechanism for the formation of an iodine-polymer complex, but does not address or consider a relationship between color intensity of the complex and iodine concentration. Furthermore, the Zwick publication fails to suggest any utility of a colored iodine-polymer complex, in particular in an assay for peroxide concentration in aqueous solution.
Various patents are directed to the starch iodine complex as the indicator in an assay of a test sample. U.S. Pat. Nos. 3,814,668 and 4,303,753 disclose the use of potassium iodide as a redox indicator in the presence of polyvinylpyrrolidone (PVP) for detection of glucose in urine. Such an indicator system is used in the DIASTIX.RTM. urine glucose test strip, marketed by Bayer Diagnostics, Elkhart, Ind. The maximum color intensity for glucose concentration is 2000 mg/dL, which is stoichiometrically equivalent to 111 mM hydrogen peroxide, and which, in turn, is equivalent to 3777 mg/L or 0.37% by weight of hydrogen peroxide. Because only a fraction of glucose in the test sample is actually reacted to release hydrogen peroxide, the indication is responding to an actual peroxide concentration that is probably much less than the calculated level of 0.37% by weight.
Similarly, U.S. Pat. No. 4,621,049 discloses the use of iodide ion and PVP as an indicator in an assay for measuring a glucose concentration as high as 10,000 mg/dL. In such a assay, however, the ability to measure a high range of glucose is achieved by modulating enzyme reactivity through a borate buffer at an alkaline pH, as disclosed in the U.S. Pat. No. 5,217,691. The indicator system, again, is responding to only a fraction of glucose in the test sample. Therefore, the indicator used in DIASTIX.RTM. is responding only to low levels of hydrogen peroxide, and does not disclose or suggest an ability to respond to a peroxide concentration of 0.1%, by weight, or higher. Other patents directed to iodide/iodine indicators are U.S. Pat. Nos. 4,181,500 and 4,992,296.
To date, no known single assay is available to assay a wide peroxide concentration range because the large peroxide concentration difference makes detection and differentiation between concentration levels difficult. The present invention is directed to providing an assay for peroxide that is capable of measuring peroxide concentration over the range of 0% to about 4%, and especially about 0.1% to about 4%, by weight, without diluting the test sample.
The present invention, therefore, is directed to an assay method and device that can be used to test sample containing 0% to 4% or more of peroxide, without diluting the solution. As illustrated hereafter, the present test strips have a good sensitivity and a wide detection range with a continuous color response from 0.1% to over 4%, by weight, peroxide, without diluting the test sample.
The present method of assaying for peroxide in an aqueous test sample yields trustworthy and reproducible results by utilizing an indicator reagent composition that undergoes a color transition in response to peroxide concentration, and not as a result of a competing chemical or physical interaction, such as a preferential interaction with another test sample component. Additionally, the method and composition utilized in the peroxide assay does not adversely affect or interfere with any other test reagent pads that are present on a multiple test pad strip.
In accordance with the present invention, an indicator reagent composition can be incorporated into a carrier matrix to provide sufficient sensitivity and color differentiation to assay for peroxide concentration over the range of 0% to greater than about 4%, and particularly about 0.1% to greater than about 4%, by weight, without sample dilution. In addition, although dry phase test strips have been used to assay for peroxide concentration, no dry phase test strip has been used to quantitatively assay an undiluted test sample for peroxide at the high concentration range.